1. Field of the Invention
The present invention relates to subscriber line/trunk circuits in general, and to an improved analog subscriber line/trunk circuit wherein a microcomputer controlled line circuit for each line or group of lines in a multiple subscriber system compensates for differing transmission characteristics from line to line without the signal distortions or including complex and costly measurement and monitoring equipment of the prior art. All functions are accomplished by the present subscriber circuit, without signal degradation, by utilizing a microcomputer per line or one microcomputer for a plurality of lines to perform the foregoing and associated digital filtering for the subscriber circuit of the present invention.
2. Description of the Prior Art
In the prior art, the problem of differing impedance and transmission characteristics of multiple line types has been approached by balancing the line impedance between the subscriber set and the switching network by insertion of components on a per line basis to achieve a "compromise" characteristic. From a loss viewpoint, this resulted in a degradation of the analog line signal of two decibels in the transmission direction and two decibels in the receiving direction. Further, if this problem is avoided by individually balancing each line, testing must be accomplished at each line, which is both expensive and time consuming. In accordance with one aspect of the present system, the line balancing for each line is accomplished by means of an automatic computation in a data processor, such as a microcomputer, by sending a tone on the line, measuring the tone upon its return and storing data for use in the system. In accordance with another aspect of the present invention, digital filters are used to compensate for attenuation with frequency and reduce the magnitude of unwanted return signals to an acceptable level. Heretofore, subscriber circuits have utilized two-wire switching, which does not require two-wire to four-wire conversion circuits, thus no undesired signal return is generated. In contrast, the padding by subtraction technique of the present invention achieves the foregoing programmably using digital filters to compensate for variation in transmission parameters. Also, in accordance with the present invention, bulky components, such as audio transformers and electro mechanical relays are eliminated. Digital filters per se are well known in the prior art. A description of the general design of such filters may be found in: Digital Processing of Signals, B. Gold and C. Rader, Lincoln Laboratory Publication, Mc Graw-Hill, 1969.